Chapter 16.19
GEOLOGICALLY HAZARDOUS AREAS
Sections:
16.19.035 Description of specific hazard areas.
16.19.040 Submittal requirements.
16.19.060 Exempt uses and activities.
16.19.010 General authority.
This chapter is adopted under the authority of RCW 36.70A.050. (Ord. 2232 § 7 (Exh. A), 2009).
16.19.020 Purpose.
It is the declared policy of the city of Centralia to encourage land uses that are compatible with underlying geological conditions through the use of appropriate engineering, design and construction practices. It is also recognized that at times even the best of efforts to properly design and apply technology will not adequately reduce the risks of geological hazards. In these instances, areas of extreme geological instability are to be avoided as sites for development and placement of structures. (Ord. 2232 § 7 (Exh. A), 2009).
16.19.030 Applicability.
Geologically hazardous areas include areas determined to be susceptible to erosion, sliding, earthquake, or other geological events. They pose a threat to the health and safety of citizens when incompatible development is sited in areas of significant hazard. Areas susceptible to one or more of the following types of hazards shall be designated as geologically hazardous areas:
A. Erosion hazard;
B. Landslide hazard;
C. Seismic hazard;
D. Volcanic hazard; and
E. Other geological events including mass wasting, debris flows, rock falls, and differential settlement. (Ord. 2232 § 7 (Exh. A), 2009).
16.19.035 Description of specific hazard areas.
A. Erosion Hazard Areas. Erosion hazard areas are at least those areas identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a moderate to severe, severe, or very severe rill and inter-rill erosion hazard.
B. Landslide Hazard Areas. Landslide hazard areas are areas potentially susceptible to landslides based on a combination of geologic, topographic, and hydrologic factors. They include areas susceptible to landslides because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors. Examples of these may include but are not limited to the following:
1. Those areas delineated by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having severe limitation for building site development.
2. Those areas mapped by the Department of Natural Resources (slope stability mapping) as unstable (“U” or class 3), unstable old slides (“UOS” or class 4), or unstable recent slides (“URS” or class 5).
3. Areas designated as quaternary slump, earth flows, mudflows, lahars, or landslides on maps published by the U.S. Geological Survey or Department of Natural Resources.
4. Areas with slopes steeper that fifteen percent and springs or groundwater seepage.
5. Areas that have shown movement during the Holocene epoch (from ten thousand years ago to present) or that are underlain or covered by mass wastage debris of that epoch.
6. Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials.
7. Slopes having gradients steeper than eighty percent subject to rock fall during seismic shaking.
8. Areas potentially unstable because of rapid stream incision, stream bank erosion, and undercutting by wave action.
9. Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding.
10. Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least ten feet of vertical relief.
C. Seismic Hazard Areas. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface faulting. Ground shaking is the primary cause of earthquake damage in Washington. The strength of ground shaking is primarily affected by:
1. The magnitude of the earthquake;
2. The distance from the source of an earthquake;
3. The type of thickness of geologic materials at the surface; and
4. The type of subsurface geologic structure.
D. Volcanic Hazard Areas. Volcanic hazard areas are subject to pyroclastic flows, lava flows, debris avalanche, inundation by debris flows, lahars, mudflows, or related flooding resulting from volcanic activity.
E. Settlement and soil liquefaction conditions occur in areas underlain by cohesionless, loose, or soft-saturated soils of low density, typically in association with a shallow groundwater table. (Ord. 2232 § 7 (Exh. A), 2009).
16.19.040 Submittal requirements.
For developments within geologically hazardous areas, a critical areas analysis shall be prepared as part of the development permit application. A geotechnical report will also need to be conducted by a geotechnical engineer or geologist, licensed in the state of Washington, with experience analyzing geologic, hydrologic, and groundwater flow systems. The following will be required:
A. General development application form.
B. Site and construction plans or maps showing:
1. Location of proposed project in relation to abutting public and private streets;
2. The total acreage, number of lots, and the proposed density for the development;
3. The location and approximate size, in square feet, of the proposed lots. The site plan must identify among other things, all slopes thirty percent or greater and the square footage and dimensions of the buildable area(s);
4. The type and extent of geologic hazard areas, and any other critical areas and buffers on, adjacent to, or within two hundred feet of the proposal;
5. Proposed development, including location of existing and proposed structures, fill, storage of materials, drainage facilities, with dimensions indicating distances to the floodplain;
6. A topographic contour map, tied to a land-based survey, with coloration, shading or hatching indicating areas within the development site with slopes of less than ten percent, ten and twenty percent, areas between twenty and thirty percent, and areas of thirty percent or greater, with contour lines spaced no less than two feet apart vertically;
7. Proposed location of structures in relationship to all geologic hazard and other critical areas; and
8. Grading plan shall show existing and proposed elevation contours including the location and depth of all proposed cuts and fills of the finished earth surfaces using a contour interval of two feet or less. Access or haul road location, treatment, maintenance requirements and the limits of disturbance shall be included. Clearing limits and the proposed area to be graded shall be clearly delineated on the plan and the area amount stated in square feet.
C. General geotechnical/geological report, which shall include the following components unless the director determines a specific component is not applicable to the proposal. The report shall contain a geotechnical analysis including a detailed description of the project, its relationship to geologic hazard(s) and its potential impact upon the hazard area, the subject property and affected adjacent properties.
1. Soil characteristics component shall include data regarding the nature, distribution, and strength of soils within the development area as well as:
a. Unified classification of all soils with liquid limit, shrink-swell potential, and general suitability for development;
b. Estimate of the normal highest elevation of the water table;
c. Flood history and potential, proximity to known flood plain area and drainage channels, springs, and other hydrological features;
d. Liquefaction analysis;
e. An assessment of the geologic characteristics and engineering properties of the soils, sediments, and/or rock of the project area and potentially affected adjacent properties;
f. A review of the site history regarding landslides, erosion, and prior grading.
2. Vegetation component shall include:
a. Location and identification of existing vegetation;
b. The vegetation to be removed and method of disposal;
c. The vegetation to be planted; and
d. Slope stabilization and erosion measures to be installed.
3. Geologic conditions component shall be:
a. A description of the surface and subsurface geology, hydrology, soils, and vegetation found in the project area and in all hazard areas addressed in the report;
b. A detailed overview of the field investigations, published data and references; data and conclusions from past assessments of the site; and site specific measurements, test, investigations, or studies that support the identification of geologically hazardous areas; and
c. A description of the vulnerability of the site to seismic and other geologic events.
4. Recommended Minimum Buffer and Building Setback. The report shall make a recommendation for the minimum no-disturbance buffer(s) and minimum building setback(s) from any geologic hazard areas based upon the geotechnical analysis.
5. Incorporation of Previous Study. Where a valid geotechnical report has been prepared within the last five years for a specific site, and where the proposed land use activity and surrounding site conditions are unchanged, said report may be incorporated into the required report if approved in advance by the director. The applicant shall submit any such reports to review any changed environmental conditions associated with the site.
D. Specific technical reports for geologic hazards must meet the following. Reports for two or more types of critical areas must meet the additional report requirements for each type of relevant type of critical area as specified herein:
1. Erosion and Landslide Areas. The technical information for an erosion hazard or landslide hazard area(s) shall include the following information:
a. A copy of the site plan for the proposal showing:
i. The height of slope, slope gradient, and cross-section of the project area;
ii. The location of springs, seeps, or other surface expressions of groundwater on or within two hundred feet of the project area or that have potential to be affected by the proposal; and
iii. The location and description of surface water runoff features.
b. Hazards Analysis. The hazards analysis component of the critical areas report shall specifically include:
i. A description of the extent and type of vegetative cover;
ii. A description of subsurface conditions based on data from site-specific explorations;
iii. Descriptions of surface and groundwater conditions, public and private sewage and disposal systems, fills and excavations, and all structural improvements;
iv. An estimate of slope stability and the effect construction and placement of structures will have on the slope over the estimated life of the structure;
v. An estimate of the bluff retreat rate that recognizes and reflects potential catastrophic events such as seismic activity or a one hundred year storm event;
vi. Consideration of the run-out hazard of landslide debris and/or the impacts of landslide run-out on down slope properties;
vii. A study of slope stability including an analysis of proposed cuts, fills, and other site grading;
viii. Recommendations for building site limitations; and
ix. An analysis of proposed surface and subsurface drainage, and the vulnerability of the site to erosion.
c. Geotechnical Engineering Report. The technical information for a project within a landslide hazard area shall include a geotechnical engineering report prepared by a licensed engineer licensed in the state of Washington that presents engineering recommendations for the following:
i. Parameters for design of site improvements including appropriate foundations and retaining structures. These should include allowable load and resistance capacities for bearing and lateral loads, installation considerations and estimates of settlement performance;
ii. Recommendations for drainage and subdrainage improvements;
iii. Earthwork recommendations including clearing and site preparation criteria, fill placement and compaction criteria, temporary and permanent slope inclinations and protection, and temporary excavation support, if necessary; and
iv. Mitigation and adverse site conditions including slope stabilization measures and seismically unstable soils, if appropriate.
d. Erosion and Sediment Control Plan. For any development proposal on a site containing an erosion hazard area, an erosion and sediment control plan is required. The erosion and sediment control plan shall be prepared in compliance with requirements set by the Storm Water Management Plan for Western Washington as adopted by the city of Centralia.
e. Drainage Plan. The technical information shall include a drainage plan for the collection, transport, treatment, discharge, and/or recycle of water prepared in accordance with the Storm Water Management Plan for Western Washington as adopted by the city of Centralia.
f. Mitigation Plans. Hazard and environmental mitigation plans for erosion and landslide hazard areas shall include the location and methods of drainage, surface water management, locations and methods of erosion control, a vegetation management and/or replanting plan, and/or other means for maintaining long-term soil stability.
g. Monitoring Surface Waters. If there is significant risk of damage to downstream from receiving waters due to potential erosion from the site, based on the size of the project, the proximity to receiving waters, or the sensitivity of the receiving waters, the technical information shall include a plan to monitor the surface water discharge from the site. The monitoring plan shall include a recommended schedule for submitting monitoring reports to the city.
2. Seismic Hazard Areas. A critical area report for a seismic hazard area shall also meet the following requirements:
a. The site map shall show all known and mapped faults within two hundred feet of the project area or that have potential to be affected by the proposal;
b. The hazards analysis shall include a complete discussion of the potential impacts of seismic activity on the site, such as forces generated and fault displacement; and
c. A geotechnical engineering report shall evaluate the physical properties of the subsurface soils, especially the thickness of unconsolidated deposits and their liquefaction potential. If it is determined that the site is subject to liquefaction, mitigation measures appropriate to the scale of the development shall be recommended and implemented.
3. Volcanic Hazard Areas. A critical area report for a volcanic hazard area shall also meet the following requirements:
a. Site Plan. The site plan shall show all areas within two hundred feet of the project area that have potential to be affected by pyroclastic flows, lahars, or mud and debris flows derived from volcanic events;
b. Hazards Analysis. The hazards analysis shall include a complete discussion of the potential impacts of volcanic activity on the site; and
c. Emergency Management Plan. The emergency management plan shall include plans for emergency building exit routes, site evacuation routes, emergency training, notification of local emergency management officials, and an emergency warning system.
E. Mitigation of Long-Term Impacts. When hazard mitigation is required, a mitigation plan shall specifically be developed to address how the activity maintains or reduces the pre-existing level of risk to the site and adjacent properties on a long-term basis (equal to or exceeding the projected life span of the activity or occupation). Proposed mitigation techniques shall be considered to provide long-term hazard reduction only if they do not require regular maintenance or other actions to maintain their function. Mitigation may also be required to avoid any increase in risk above the pre-existing conditions following abandonment of the activity. (Ord. 2232 § 7 (Exh. A), 2009).
16.19.050 Regulations.
A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that:
1. Will not increase the threat of the geological hazard to adjacent properties beyond pre-development conditions;
2. Will not adversely impact other critical areas;
3. The development will not decrease slope stability on adjacent properties;
4. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than pre-development conditions; and
5. Are certified as safe by a qualified engineer or geologist, licensed in the state of Washington.
B. No dwelling or accessory structures shall be constructed on any steep slopes of forty percent or greater.
C. Activities on sites containing erosion or landslide hazards shall meet the specific following requirements:
1. A buffer shall be established from all edges of landslide hazard areas. The size of the buffer is discretionary and shall be determined by the director to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of the critical area report prepared in accordance with CMC 16.19.040.
a. The minimum buffer shall be equal to the height of the slope or fifty feet, whichever is greater.
b. The buffer may be reduced to a minimum of ten feet when a qualified professional demonstrates to the director’s satisfaction that the reduction will adequately protect the proposed development, adjacent developments and uses and the critical area.
c. The buffer may be increased where the director determines a larger buffer is necessary to prevent risk of damage to proposed and existing development.
2. Developments within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this title. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development standards are:
a. The proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. Analysis of dynamic conditions shall be based on a minimum horizontal acceleration as established by the current version of the International Building Code adopted by the city;
b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas;
c. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography;
d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation;
e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties;
f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and
g. Development shall be designed to minimize impervious lot coverage.
3. The edge of the buffer shall be clearly staked, flagged, and fenced prior to any site clearing or construction. Field marking shall be shown on the construction plans and shall remain in place, in functional condition, for the duration of construction.
4. Buffers shall be shown or described on all plats recorded in conjunction with development.
5. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited.
6. Any clearing of natural or planted vegetation within the landslide hazard area shall be prohibited, except that:
a. Up to two thousand five hundred square feet of clearing may be allowed in order to construct access roads, pedestrian paths and utility corridors provided no removal of trees greater than six inches in diameter shall occur; or
b. Upon a showing that the trees/vegetation are not necessary to preserve slope stability or reduce erosion, the director may permit a maximum of forty percent of the trees greater than six inches in diameter to be removed in conjunction with construction.
7. Clearing shall be allowed subject to approval of an excavation permit only from May 1st to October 31st of each year; provided, that the city may extend or shorten the time period on a case-by-case basis depending on actual weather conditions. All grading in all landslide hazard areas shall be stabilized by October 1st of each year and may not resume until April 1st of the following year. However, if the applicant submits documentation to substantiate that adverse impacts will not result from construction and site activity between these dates, the director may authorize certain activity.
8. Timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practice permit issued by the Washington State Department of Natural Resources.
9. Utility lines and pipes are permitted in erosion and landslide hazard areas only when the applicant demonstrates that no other practical alternative is available.
10. Point discharges from surface water facilities and roof drains onto or upstream from erosion or landslide hazard area must be:
a. Conveyed via continuous storm pipe down slope to a point where there are no erosion hazard areas downstream from the discharge;
b. Discharged at flow durations matching predeveloped conditions with adequate energy dissipation, into existing channels that previously conveyed stormwater runoff in the predeveloped state; or
c. Dispersed discharge upslope of the steep slope onto a low-gradient undisturbed buffer demonstrated to be adequate to infiltrate all surface and stormwater runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope.
11. The division of land in landslide hazard areas and associated buffers is subject to the following:
a. Land that is located wholly within a landslide hazard area or its buffer may not be subdivided. Land that is located partially within a landslide hazard area or its buffer may be divided; provided, that each resulting lot has sufficient buildable area outside of, and will not affect, the landslide hazard or its buffer; and
b. Access roads and utilities may be permitted within the landslide hazard area and associated buffers if the city determines that no other feasible alternative exists.
12. On-site sewage disposal systems, including drain fields, are prohibited within erosion and landslide hazard areas and related buffers.
13. All drainage systems and discharge points associated with actions shall be approved by the city engineer.
14. A site or portions of a site may be declared unbuildable if it is a geologically hazardous area, as described in this section, and it represents a severe risk, which cannot be successfully ameliorated by structural design.
15. Critical facilities shall not be sited within geologically hazardous areas unless there is no other practical alternative.
16. In order to increase the functional attributes of the buffer, the director may require that the applicant enhance the buffer with native vegetation.
17. The director may restrict development coverage and construction activity areas to the most level, environmentally suitable and naturally stable portion of the site. Grading activities may be restricted if necessary to ensure stability of the site.
18. Based on the information provided in the site evaluation, project plans, and geotechnical report, the director may approve, conditionally approve or deny the proposal. Any conditions shall be necessary to ensure the stabilization of the site during and/or following construction. (Ord. 2232 § 7 (Exh. A), 2009).
16.19.060 Exempt uses and activities.
A. Activities within the improved right-of-way including but not limited to construction of new utility facilities or improvements or upgrades to existing utility facilities that take place within existing improved right-of-way or existing impervious surface.
B. Operation, Maintenance or Repair. Operation, maintenance, or repair of existing structures, infrastructure improvements, utilities, public or private roads, dikes, levees, or drainage systems, that do not require construction permits, if the activity does not further alter or increase impact to, or encroach further within, the critical area or buffer and there is no increased risk to life or property.
C. Minor Utility Projects. Utility projects which have minor or short duration impacts to critical areas, as determined by the director in accordance with the criteria below, and which do not significantly impact the functions or values of a critical area(s); provided, that such projects are constructed with best management practices. Minor activities shall not result in the transport of sediment or increased stormwater. Such allowed minor utility projects shall meet the following criteria:
1. There is no practical alternative to the proposed activity with less impact on critical areas;
2. The activity involves the placement of a utility pole, street signs, anchor, or vault or other small component of a utility facility; and
3. The activity involves disturbance of no more than seventy-five square feet.
D. The director may allow the following activities within geologically hazardous areas if the activity will not increase the risk of hazard:
1. Construction of new buildings with less than two thousand five hundred square feet of floor area or roof area, whichever is greater, and which are not residential structures or used as places of employment or public assembly;
2. Additions to existing residences that are two hundred fifty square feet or less;
3. Installation of Fences. Fencing may be dark brown, dark green or black vinyl coated chain link to blend in with the native landscaping. In no case shall the following types of fences be allowed: uncoated chain link, masonry, block, wood or other sight obscuring material; and
4. Trails. A trail may be constructed for access subject to the following conditions:
a. No cut or fill of the hillside may be in excess of two feet. All cuts or fills shall be properly retained;
b. The trail should follow a meandering course and not use a direct line pathway to the desired location. Where possible, the trail should follow the natural contours of the hillside;
c. The trail shall be screened with natural landscape material; and
d. The trail plan shall be submitted to the director for review and must be approved prior to any construction and/or hillside cuts. (Ord. 2232 § 7 (Exh. A), 2009).